The present invention relates in general to determining co-channel interference levels for wireless transmissions, and, more specifically, to measuring a desired/undesired ratio using portable equipment that does not require disruption of broadcasting or complicated test equipment or procedures.
In connection with obligations of licensees of wireless broadcasting services, it often becomes necessary to measure various signals at potential receiving locations in order to comply with requirements designed to minimize interference between different broadcasters. For example, in the Broadband Radio Service (BRS) authorized in a 2.5 GHz band by the Federal Communications Commission in the United States, a transition is being conducted wherein licensees of the Multipoint Distribution Service (MDS) are being reassigned to frequencies in the BRS. The BRS has been used to broadcast analog television (i.e., video) signals. Some new licensees in the BRS will operate on the same frequencies as existing licensees with the band. Licensees at the same frequencies will operate in respective service areas, but the potential for co-channel interference still exists and the FCC has specified certain interference requirements to be met. More specifically, the FCC requires that, as measured at a particular receiving site, the co-channel desired/undesired (D/U) ratio for a protected (i.e., previously existing) licensee must be at least the lesser of either 45 dB or the actual D/U ratio at the receiving site for the previously existing licensee prior to the transition minus 1.5 dB.
It is known that D/U ratio measurements can be done by first measuring the received power of a desired signal and then shutting off the desired transmitter and measuring the level of any undesired signal that may be present. This type of testing creates problems because it may be necessary to shut off the transmitter repeatedly or for noticeably long periods, resulting in interruption of programming to viewers being served by the BRS licensee (which may be a cable television provider, for example). In addition, the coordination required if multiple receive sites are being transitioned can be difficult and time consuming.
Difficulties arise when attempting to conduct measurements of desired and undesired power when both signals are present simultaneously. Typically, the undesired signal falls within a well-defined window relative to the desired signal. Considering the BRS service, the frequency difference between the two signals will be between 0 kHz and 11 kHz, and the D/U need only be measured down to 45 dB. As the frequency separation between the two transmitters approaches zero, extremely high resolving capability would be required in any measuring equipment. This situation can be improved by shifting the frequency of the desired transmitter to increase the separation, but even with frequency shifting the proximity of the desired and undesired carrier frequencies as well as the complex voltage of the active video signal makes the D/U measurement virtually impossible using standard test equipment. Relatively expensive equipment and/or highly skilled test operators have been required.